VJ 1204 -- yellow printing more than yellow?

Does anyone know why my 1204 prints magenta and cyan as well as yellow in a fill that's supposed to be all yellow?? I'm trying to match an off white, and in doing some test fills, I designed several fills, some of which I assigned 3 - 15% yellow fills, no C, M, or K. I'm using Flexi Pro 8.5, assigning the fills right into the editor.

Notice the images below, about the best I can explain. This fill is assigned 0C0M8Y0K. But it has several specks of magenta, and even more of cyan. Thus giving a green look, which I CAN'T have. Second pic is really poor, but might help explain a bit.

Any thoughts on this? I really don't know too much about the printers (too obvious?? ), maybe I'm missing something really simple.
Thanks!!

 

Very much so

I simply filled several small boxes with various process fills. Thanks for the tip, I haven't gone that far yet (honestly I'm not sure what it's set at right now). I do have the settings set for the particular vinyl it's printing on. I'll give that a try.

 

8 years later I can't remember what I was even printing then But I think now the same may be true as for printing *pure* black. I use Flexi, so under Color Settings, under Rendering Intent, choose "pure hue" for the appropriate color. I haven't tested on yellow and can't remember what I even did but it may work the same for yellow. I would have replied sooner but I didn't get email notifications for this again for some reason, until I now looked.......

 

That is amazing, I've never seen that before until now. Thank you!!!! I'll assume the tolerance slider on the output will allow for slight variations. I'll give this a try.